unit 4 Central Neural Mechanisms in Movement

Central Neural Mechanisms in Planning and Initiating Movement

• Movement initiation involves a complex interplay between various brain regions, including the cortex, basal ganglia, cerebellum, and brainstem.

• The motor cortex is crucial for planning and executing voluntary movements. Different areas within the motor cortex are specialized for controlling specific muscle groups and body parts.

• The premotor cortex plays a critical role in planning and sequencing movements, receiving input from areas involved in sensory processing and higher-level cognitive functions.

• The supplementary motor area (SMA) is also involved in planning, particularly for complex, internally generated movements. It also plays a role in the initiation and coordination of sequences.

• The basal ganglia are a group of interconnected subcortical nuclei that play multifaceted roles in motor control, including regulating the force and speed of movements, inhibiting unwanted movements, and contributing to the initiation of actions. Parkinson's disease is a neurological disorder that affects the basal ganglia, leading to motor impairments.

• The cerebellum is vital for motor learning, coordination, and error correction during movement execution. It receives sensory feedback from the body and the motor commands from the cortex to refine and adjust ongoing motions. Cerebellar damage can result in impaired motor coordination and balance.

• The brainstem contains essential motor nuclei that control vital reflexes, posture, and muscle tone. Many cranial nerves originate in the brainstem and control eye movements, facial expressions, and other cranial muscles. The brainstem integrates information from numerous cortical and subcortical sources to adjust motor commands based on sensory input.

• Planning and initiation of movement require the integration of sensory information, such as proprioception (sense of body position) and vestibular input (balance). Processing this information allows the brain to create appropriate motor commands.

• Motor plans can be influenced by various cognitive factors such as motivation, attention, and decision-making processes. Higher-level cognitive functions interact with the motor system to adjust movement strategies based on current context and goals.

• Neural pathways for different types of movements are specialized, some for simple, reflexive actions and others for intricate, learned actions.

• Individual neural circuits form complex networks that generate specific patterns of muscle activity.

• Sensory feedback loops play a crucial part in the continuous adjustment of movements. The brain constantly processes sensory information to refine motor commands and adapt to changing conditions.

• Motor learning involves changes in neural circuits, allowing for improved motor performance over time. Practice strengthens synaptic connections and refines motor control.

• Neurotransmitters, such as dopamine, play critical roles in modulating movement initiation and control. Imbalances in neurotransmitter systems can impact motor functions.

Specific Neural Pathways

• Direct pathway: A relatively direct pathway from the motor cortex to the spinal cord for initiating quick, precise movements.

• Indirect pathway: A more complex pathway involving several intermediate structures, such as the basal ganglia and cerebellum that influence movement, allowing for more complex and adaptable actions.

• Sensory feedback pathways: These pathways provide constant sensory information about the body's position and movement, allowing for corrections and adjustments.

Disorders Affecting Movement

• Neurological disorders, such as Parkinson's disease, Huntington's disease, and cerebellar ataxia, disrupt the normal functioning of these neural mechanisms, leading to various motor impairments.

• Stroke can also result in deficits in movement initiation and execution in affected limbs, often requiring rehabilitation protocols to regain lost function.

• 1. Intermuscular Coordination

  • Definition: The coordination of multiple muscle groups and body segments to perform a task efficiently and effectively.
  • Roles of Muscles:
    • Agonist: The primary muscle performing the movement.
    • Antagonist: Opposes the agonist to control or decelerate the movement.
    • Stabilizer: Holds a body part in place to allow the primary movement.
    • Neutralizer: Counters unwanted movements caused by other muscles.

  Key Measurements:

  • Electromyography (EMG):
    • Detects muscle electrical activity (muscle action potential) using surface or inserted electrodes.
    • Measures efficiency and activation patterns of agonists and antagonists.
  • Biomechanical Efficiency:
    • Evaluates how effectively muscles coordinate to minimize energy waste.

  Training Adaptations:

  • Untrained individuals exhibit less coordination:
    • Overactivation of antagonists reduces movement efficiency.
  • Trained individuals demonstrate better synchronization between agonists and antagonists:
    • Agonists are more active, and antagonists are less active during task execution.

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  2. Intramuscular Coordination

  • Definition: The coordination within a single muscle, focusing on how motor units (MUs) work together to produce force.

  a. Motor Unit Recruitment and Firing Rate:

  • Recruitment:
    • Involves activating more motor units as force demand increases.
    • Predominates in the low force range.
  • Rate Coding:
    • Refers to increasing the firing rate of active motor units for additional force.
    • Predominates in the high force range.
  • Factors Affecting Recruitment vs. Rate Coding:
    • Small Muscles (e.g., hands): Full MU recruitment at ~30% Maximal Contraction (MC), relying on rate coding for higher forces.
    • Large Muscles (e.g., quadriceps): Recruitment dominates until ~80-90% MC.

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  b. Discharge Patterning:

  • Smooth Contractions (Most Tasks):
    • Motor units alternate activity to maintain steady force and prevent fatigue.
  • Maximal Force (High-Intensity Tasks):
    • Motor units fire synchronously to generate maximum force output.

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  c. Compartmentalization:

  • Definition: Division of muscle fibers into smaller, independently controlled compartments within a single muscle.
  • Factors Influencing Compartmentalization:
    • Muscle Morphology: Fast-twitch (Type II) vs. slow-twitch (Type I) fibers.
    • Neural Recruitment: Specific compartments activate during specific tasks.
    • Biomechanical Function: Compartments may vary in activation based on movement angle or load.
  • Example: The deltoid muscle compartments:
    • Anterior: Active during flexion.
    • Medial: Active during abduction.
    • Posterior: Active during extension.

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  3. Adaptations of Motor Unit Behavior to Training

  • Strength and Power Training:
    • Increases the maximal level of motor unit activation to 100% if not previously maximal.
    • Enhances motor unit firing rates.
    • Causes earlier recruitment of high-threshold motor units.
    • Enables quicker attainment of high firing rates for faster force development.
  • Outcomes of Training:
    • Increased force output.
    • Improved movement efficiency.
    • Greater neuromuscular coordination.

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  4. Applications

  Example 1: Sprinting

  • Intermuscular Coordination: Agonists (quadriceps) and antagonists (hamstrings) work together for optimal stride efficiency.
  • Intramuscular Coordination: High-threshold motor units in the quadriceps are recruited early to generate explosive power.

  Example 2: Weightlifting

  • Intermuscular Coordination: Stabilizers (core muscles) hold the body steady while primary movers (e.g., gluteus maximus) execute the lift.
  • Intramuscular Coordination: Synchronized firing of motor units enhances strength and power during the lift.

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  Key Takeaways

  1. Intermuscular Coordination:

     • Focuses on coordination between different muscles.
     • Enhanced by reducing unnecessary antagonist activity and optimizing agonist activation.

  2. Intramuscular Coordination:

     • Focuses on motor unit recruitment and firing within a single muscle.
     • Improved through strength and power training, leading to faster and more powerful contractions.

  3. Practical Implication:

     • Optimizing both intermuscular and intramuscular coordination is crucial for enhancing performance in sports and functional movements​(PHYL4518 - Wk5 - Oct 2 …).

     Major Roles of Open-Loop Organizations
     • Determine which muscles contract
     when, how forcefully, and for how long
     • To organize the many degrees of
     freedom of the muscles and joints into a
     single unit
     • To determine postural adjustments
     necessary to support the upcoming
     action
     • E.g. bicep pull experiment
     • To modulate the many reflex pathways
     to ensure that the movement goal is
     achieved

     Control of Movement:
     Open-loop vs. closed loop
     • Two ways in which movements could be controlled
     • Open-Loop Control: A type of system control in which instructions for the
     effector system are determined in advance and run off without feedback
     • Closed loop Control: A type of system control involving feedback, error
     detection, and error correction that is applicable to maintaining a system
     goal.

     Closed-Loop Control Systems:
     General Concept Example
     • Desired state is set (20oC)
     • Sensory information measured and compared
     to expected temperature
     • Any difference detected as error (e.g. too
     cold)
     • Error transmitted to executive to decide what
     to do to eliminate error (e.g. decide to turn on
     furnace)
     • Command sent to effector (furnace turns on)
     • The action returns the system to the desired
     state (20oC)
     • This information is sent to the executive, and
     the cycle continues (e.g. furnace turns on and
     off all day to maintain house temperature)
     Desired state:
     20oC
     Executive
     System
     Effector
     System
     Comparato
     r
     Actual state
     Error
     Sensory info
     Negative feedback loop!

     Closed-Loop Control in Human
     Performance
     • Reaching to pick up cup
     • Visual info about hand’s position relative to cup
     represents feedback (i.e. information about the
     movement outcome)
     • Difference in hand location and desired location
     represent errors
     • Executive determines correction and modifies an
     effector
     • Most movements have several feedback
     sources

     Closed-Loop Control in Human
     Performance
     Input
     Stimulus
     Identification
     Response
     Selection
     Movement
     Programmin
     g
     Motor
     Program
     Spinal Cord
     Movement
     Comparator
     Error
     Exteroceptive feedback
     Proprioceptive feedback
     Muscles

     Input
     Stimulus
     Identification
     Response
     Selection
     Motor
     Program
     Spinal Cord
     Movement
     Comparator
     Error
     Exteroceptive feedback
     Proprioceptive feedback
     Muscles
     Closed-Loop Control: Feedforward
     Anticipated feedback
     Movement
     Programmin
     g

     Closed-Loop Control: Feedforward
     • Anticipated feedback (also called
     feedforward info)
     • Sensory consequences that are
     expected to arise
     • Why can’t you tickle yourself?
     • If anticipated feedback matches
     actual feedback, then there is
     diminished perception of sensation
     • Example 2: Force escalation
     between siblings
     • Shergill et al., 2003
     • 38% increase in force between each
     turn

     Limitations of Closed-Loop Control
     Models
     • 1. Very Slow
     • Feedback must be sent to executive,
     and information must be processed
     (seen as reaction time)
     • Example: Tracking tasks (follow a
     moving target)
     • Only about 3 corrections per second are
     possible
     • E.g. bouncing football is hard to grab

     Limitations of Closed-Loop Control
     Models
     • 2. Rapid, discrete tasks would be impossible
     under this model
     • E.g. texting, playing guitar
     • These movements occur too quickly to process info
     before the movement is complete
     • Therefore, these movements must be programed in
     advance

     Motor Program Theory:
     Closed-Loop and Open Loop Control
     • Closed loop = Open loop with feedback
     • In most tasks, motor behavior is neither
     open- nor closed-loop alone but a
     complex blend of the two
     • Slow movements  Control dominated by
     feedback
     • Fast/brief movements  Open-loop
     dominates